A semiconductor epitaxial wafer obtained by forming an epitaxial layer on a semiconductor wafer is used as a device substrate for producing various semiconductor devices, such as a metal-oxide-semiconductor field-effect transistor (MOSFET), dynamic random access memory (DRAM), a power transistor, and a back-illuminated solid-state imaging device.
For example, a back-illuminated solid-state imaging device can directly take outside light into a sensor to capture a sharper image or video even in a dark place and the like, by arranging a wiring layer and the like below a sensor part. Hence, back-illuminated solid-state imaging devices are widely used in digital video cameras and mobile phones such as smartphones in recent years.
With increasing refinement and performance improvement of semiconductor devices in recent years, semiconductor epitaxial wafers used as device substrates are desired to have higher quality, in order to improve device properties. For further improvement in device properties, crystal quality improving techniques by oxygen precipitation heat treatment, gettering techniques for preventing heavy-metal contamination during epitaxial growth, etc. have been developed.
For example, JP 2013-197373 A (PTL 1) describes a technique of controlling oxygen precipitation heat treatment conditions, when performing oxygen precipitation heat treatment on a silicon substrate and then forming an epitaxial layer to produce an epitaxial wafer. With the technique described in PTL 1, the value of leakage current of the epitaxial wafer after the formation of the epitaxial layer can be limited to 1.5E-10A or less.
Moreover, we have proposed the following technique with regard to gettering, in WO 2012/15716 A1 (PTL 2): A method of producing a semiconductor epitaxial wafer comprises: a first step of irradiating a surface of a semiconductor wafer with cluster ions, to form a modifying layer formed from a constituent element of the cluster ions contained as a solid solution in a surface portion of the semiconductor wafer; and a second step of forming an epitaxial layer on the modifying layer of the semiconductor wafer.